Heretofore, various additives or methods of preparation have been utilized to improve the crack growth resistance of specific types of urethanes. However, with regard to urethane prepolymers made from polypropylene glycol reacted with MDI and cured with metal halide salt complexes of methylenedianiline, good crack growth resistance was generally not known or was unavailable.
U.S. Pat. No. 4,046,743 to Schonfeld relates to amine curable polymers or prepolymers cured with a methylenedianiline complex in the presence of a high dielectric constant catalytic compound which is relatively non-reactive with amines and isocyanates whereby increased cure rates and/or decreased cure temperatures are obtained. Examples of such high dielectric constant compounds include oleic acid, glycerin, tributyl phosphate, and the like. The urethane prepolymer is made by the reaction of a polyether, a polyol, a polyester polyol, polybutadiene diols, or combinations thereof with a polyisocyanate. This patent does not relate to blends of polyether intermediates wherein one of the polyether intermediates has ethylene oxide units therein, or to cure of MDI terminated prepolymers metal halide salt complexes of methylenedianiline in the presence of glycerine.
A brochure by DuPont "Adiprene Urethane Rubber", AP-720.21 Caytur 21, page 6, states that a method for obtaining satisfactory cure in thick sections of polymers containing high NCO content is to add a catalyst, such as glycerol, which reduces the unblocking temperature of Caytur 21. However, the prepolymer relates to Adiprene L-315 and L-767 which are polytetrahydrofuran prepolymers terminated with toluene diisocyanate. Thus, it does not contain a polyether intermediate having ethylene oxide units therein, or to cure with MDI terminated prepolymers with metal halide salt complexes of methylenedianiline in the presence of glycerine.